Conventionally, systems have been studied which apply multi-wavelength light including a plurality of optical carriers generated by an optical short-pulse laser or by amplitude modulation/phase modulation to wavelength division multiplexing (WDM) signal transmission. Such multi-wavelength light has the same spectral spacing between individual side modes so that the channels obtained by wavelength demultiplexing of the side modes have the same wavelength spacing. Accordingly, such multi-wavelength light is simpler in wavelength constellation than multi-wavelength light based on a method of preparing separate lasers for individual channels and setting wavelengths for the individual channels.
To implement a WDM signal transmission system using the multi-wavelength light, one of the important problems is the simplification of the configuration of an optical modulation circuit and its economization. FIG. 1 shows a configuration of a conventional optical modulation circuit. The multi-wavelength light generated by a multi-wavelength light source 101 undergoes wavelength demultiplexing by a wavelength demultiplexer 103, modulating by individual optical intensity modulators 105, and multiplexing by a wavelength multiplexer 107 again. The configuration as shown in FIG. 1 requires two wavelength multi-demultiplexers 103 and 107 having the same absolute value in the transmission central wavelengths. Thus, an optical modulation apparatus with a configuration as shown in FIG. 2 is proposed which includes a wavelength multi-demultiplexer 207, one or more optical intensity modulators 209, and reflecting mirrors 211 equal to the optical intensity modulators in the number (see, Japanese patent application Laid-open No. 2002-318374).
In the optical modulation apparatus as shown in FIG. 2, the multi-wavelength light, which is input to an input port 203 of an optical input section 201, passes through an input/output port 205, and undergoes wavelength demultiplexing by a wavelength demultiplexer 207, modulating by individual optical intensity modulators 209, and reflection by optical reflectors 211. Then, the reflected light rays return the paths they have came with being multiplexed again by the wavelength multiplexer 207 and output from an output port 213 of the input/output means 201. According to the system configuration, it includes only one wavelength multi-demultiplexer 207. Consequently, it can facilitate the matching of the transmission central wavelengths of the wavelength multi-demultiplexer, and reduce the cost of the system.
In either FIG. 1 or FIG. 2, the individual wavelengths have their optical power reduced by the losses of optical devices used by the wavelength multi-demultiplexer and the like. In addition, as for the system having the multi-wavelength light source and the optical modulators placed at a distance physically, the losses of fiber transmission paths linking them become nonnegligible. Since the reduction in the WDM signal power deteriorates the signal-to-noise ratio (SNR), the power must be amplified using an optical amplifier designated by the reference numeral 109 of FIG. 1 or by 215 of FIG. 2.
FIGS. 1 and 2 each show an example which amplifies the WDM signal power at once with a broadband optical amplifier that covers the entire wavelength band of the multi-wavelength light (see, Japanese patent application laid-open No. 2003-18853). The example employs a polarization independent optical amplifier that amplifies the optical intensity without depending on the polarization of the modulated light passing through the wavelength division multiplexing. Such an optical amplifier generally employs a fiber amplifier such as an erbium (Er) doped fiber amplifier (EDFA). The EDFA is an optical amplifier that amplifies the light traveling through the fiber by doping the core of the silica glass fiber with erbium ions Er3+, and by utilizing the stimulated emission in the transition proper to the ions. On the other hand, as an optical amplifier used for the optical communication, a semiconductor optical amplifier (SOA) has been developed. The SOA is an optical amplifier that amplifies the light traveling through the active layer of the semiconductor by the stimulated emission by reducing the reflectance of end faces of the cavity of the semiconductor laser.
Although both types of the optical amplifiers have a broad gain bandwidth of 30 nm or more, they differ greatly in the lifetime of carriers in the excited level. Since the EDFA has the gain broadening established by the transition from a plurality of discrete excitation energy levels, it has a long carrier lifetime of an order of milliseconds, and uneven gain broadening. In contrast with this, the SOA has a short carrier lifetime of an order of nanoseconds, and the gain broadening can be considered as uniform. Generally, the optical amplifier operates in the saturation region of the gain to obtain large output. When the optical amplifier with the uniform gain broadening amplifies a plurality of different signal wavelengths in the saturation region of the gain, the individual wavelengths scramble for the gain, which causes crosstalk between the channels and degrades the signal waveform. Accordingly, fiber amplifiers such as the EDFA are usually used to amplify the WDM signal collectively as described above. However, comparing the SOA that excites the semiconductor by injection current with the EDFA that includes a semiconductor laser for outputting pumping light, a doped fiber doped with erbium or the like, and a coupler for coupling the pumping light to the doped fiber, the SOA is far economical from the viewpoint of the number of components. In particular, the SOA is more suitable for amplifying a single signal wavelength.
To amplify the WDM signal collectively using the fiber amplifier, it is essential to increase the power of the optical amplifier to compensate for the optical losses caused by optical components such as the wavelength multi-demultiplexer and optical intensity modulators. However, a broadband, high-power light amplifier covering the entire wavelength band of the multi-wavelength light is very expensive even if used alone. Accordingly, depending on the wavelength bandwidth and output required, a configuration that amplifies the wavelengths individually by the SOAs can sometimes implement the optical modulation circuit more cheaply than the configuration using the fiber amplifier.
In addition, the SOA has the following advantages.                The SOA is applicable as a modulator by varying the injection current in response to a modulation signal.        The SOA can be integrated with an electro absorption modulator (EA modulator) and the like.        
Next, typical configuration examples of the optical modulation apparatus using the SOAs will be described.